Roll diffusion bonding method

ABSTRACT

This disclosure pertains to a method of facilitating removal of magnetically responsive filler elements from an assembled and bonded non-magnetically responsive workpiece by subjecting the workpiece to a magnetic force in an amount sufficient to distort and loosen the filler elements.

United States Patent Conn, Jr. et al.

[54] ROLL DIFFUSION BONDING METHOD [72] inventors: Charles Conn, Jr.,Manhattan Beach; Robert P. Neumann, Torrance, both of Calif.

[73] Assignee: North American Rockwell Corporation [22] Filed: Jan. 27,1969 211 Appl. No.: 801,913

Related US. Application Data [62] Division of Ser. No. 605,419, Dec. 28,1966, Pat. No.

[52] US. Cl ..29/42l, 72/56, 72/706, 29/423, 29/427, 29/471.]

[451 June 13, 1972 [51] Int. Cl. ..B23p 17/00 [58] Field ofSearch..29/423, 426, 427, 42l M, 471.1

Primary Examiner-John F. Campbell Assistant Examiner-Richard BernardLazarus AttorneyWilliam R. Lane, Charles F. Dischler and Harold H. Card,Jr.

571 ABSTRACT This disclosure pertains to a method of facilitatingremoval of magnetically responsive filler elements from an assembled andbonded non-magnetically responsive workpiece by subjecting the workpieceto a magnetic force in an amount sufficient to distort and loosen'thefiller elements.

4 Claims, 6 Drawing Figures PATENTEBJUH 13 m2 FIG.2

ATTORNEY ROLL DIFFUSION BONDING METHOD This is a division of applicationSer. No. 605,419 filed Dec. 28, 1966, now US. Pat. No. 3,444,608.

This invention pertains to a method for mass production of articles byroll diffusion bonding, especially but not exclusively planar articlessuch as lightweight panels of thin-walled sandwich type construction.More particularly, this invention concerns a method for achievingimproved dimensional accuracy and structural integrity in the final partby rapid, economical and practical means adapted for modern massproduction of articles such as the mentioned type panels.

Lightweight panels of sandwich type are useful in structures involvinggreat strength at high temperature wherein economy of weight is a primeconsideration, such as in the fabrication of high speed aerial or spacemissiles and vehicles. In addition, great precision in such workpiecesis necessary in order to assemble the component parts of a massivevehicle or missile. Such panels typically comprise two confronting outerface sheets uniformly spaced apart in substantially parallelrelationship with a plurality of internal upstanding ribs or angularcorrugations permanently joined to both stated sheets. Of the variousmaterials suitable for the stated use, titanium is widely used becauseof its high strength and light weight, although other and differentmaterials may be used in the article produced by the method disclosedherein. The known prior art includes various methods for fabricatingtitanium sandwich type panels by roll diffusion bonding of the internalribs to the face sheets starting with a relatively thick workpiece paneland applying heat and pressure through rollers to form permanent solidstate bonded joints between the workpiece components, and incidentallyeffecting a substantial reduction in panel thickness with commensurateelongation of the same. To control and distribute the loads anddeformation effects during such rolling, filler material in the typicalform of mandrels or the like of suitable material such as mild steel areused to occupy the space between the internal ribs or corrugations, thesteel filler material being compressed and elongated along with thetitanium workpiece components during the rolling operation. The mandrelsand workpiece are typically encased within a steel envelope prior to therolling operation, and the entire pack subjected to rolling force.

In the foregoing process, it has been a persistent and predominantproblem to achieve defect-free panels having the requisite size,strength and structural integrity for use in vehicles and missiles ofthe type mentioned. A principal cause of the stated problem isnon-uniform load or stress effects which produce localized wrinkling,buckling or tearing of the face sheets or delicate internal ribstructure during the rolling operation. Moreover, since the steelmandrels or other filler and envelope materials used in the pack aredeformed and joined to the titanium workpiece surfaces within theenvelope during the rolling operation, the difficulty and high cost ofremoving such mandrels and envelope materials from the workpiece withoutdamaging the latter is a major deterrent to the widespread adoption ofroll diffusion bonded panels of the type described above. Thus, the highincidence of defects from the stated causes is directly associated witha high percentage of rejection of final articles, with the result thatproduction costs for panels of the mentioned type are prohibitive inmany cases.

Accordingly, it is a principal object of the invention in this case toprovide a strong, dimensionally accurate and distortion-free rolldiffusion bonded panel having improved structural integrity.

It is another object of this invention to provide a method for makingroll difiusion bonded articles in accordance with the above statedobjects which may be practiced with improved ease, economy and rapidity.

It is another object in this case to provide a method for roll diffusionbonded objects having improved accuracy in controlling the structuralquality of the article during its fabrication, thus avoiding a highincidence of rejection in the mass production of such articles.

It is a further object in this case to provide a method of making anarticle as mentioned in the above objects by means permitting removal ofenvelope materials and mandrels or the like from such article withimproved rapidity, ease, economy and lessened risk of deleteriouseffects upon the article as a result of the mandrel and envelope removalsteps.

Other objects and advantages will become apparent upon a close readingof the following detailed description of an illustrative embodiment ofthe inventive concept, reference being had to the accompanying drawings,wherein:

FIG. 1 is a fragmented plan view of an envelope containing workpiece andtooling components arranged according to the inventive concept in thiscase,

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1 showingthe structure thereof prior to the rolling operation,

FIG. 3 is an enlarged cross-sectional view showing a portion of thestructure from FIG. 2,

FIG. 4 is an isolated cross-sectional view corresponding generally withFIG. 2 but showing a finished article fabricated in accordance with theinventive principles disclosed herein after its removal from the toolingenvelope following the rolling operation, and with internal pressureapplied,

FIG. 5 is a fragmented view in perspective, of a workpiece removed fromits envelope but containing filler material sought to be separated fromwithin the workpiece, and

FIG. 6 shows a perspective view of a workpiece portion during a laterstep in the mandrel removal process than that depicted in FIG. 5.

Referring to FIGS. 2 and 4, for example, it may be seen that theinventive method in this case may illustratively be used in thefabrication of an article essentially comprising upper and lower facesheets 10 and 12, respectively, arranged in confronting and spaced-apartrelationship and permanently maintained in such relationship by aplurality of thin-walled ribs or webs 14 extending between the statedsheets and secured thereto by solid state diffusion bonding. Fabricationof the foregoing article begins with arrangement of the workpiececomponents in the general relationship desired for the final part asshown in FIG. 2, which may illustratively involve convergence of eachadjacent pair of rib sections 14 in alternate sequence to form nodes 16.With the arrangement thus shown, a plurality of mandrels which functionas filler material between the workpiece internal components arearranged between ribs 14 and face sheets 10 and 12 as illustrated bymandrels 18 seen in FIGS. 2-5, inclusive. Mandrels 20 and 22 areessentially half the size of mandrels 18 due to their location at eachend of the article shown in FIG. 2.

Prior to the rolling operation described below, the foregoing items 10through 22 described above are situated and arranged in the relationshipshown by FIG. 2 within a retort or envelope comprising a yoke portion ofrectangular shape shown in FIG. 1 securely joined to relatively thickand sub stantially parallel top and bottom cover plates 30 and 32 bestseen in FIG. 2. The stated yoke defines a cavity 42 and may convenientlybe formed by parallel side pieces 34 and 36 securely joined to parallelend pieces 38 and 40 by suitable means such as welding. Alternatively,the yoke may be initially formed as a single unitary member. Theenvelope plus its entire contents is herein termed the pack.

Although as stated above the selection of particular materials inpracticing the novel method disclosed herein is not critical to the useof the inventive concept, workpiece components 10, 12 and 14illustratively comprise titanium, while mandrels or filler bars 18, 20and 22 comprise mild steel. In any case, filler materials appropriatefor use with a particular workpiece material should preferably havecompression characteristics generally corresponding to those associatedwith the chosen workpiece materials at the temperatures associated withthe rolling operation. Moreover, a particular advantage is had when suchfiller materials are responsive to magnetic force in contrast withrelative insensitivity of the workpiece material to such force, forreasons appearing more clearly hereinbelow. In

connection with the preparation of a pack by preplacement of panel andfiller materials in the yoke, it has been found that a certain criticalrelationshipexists between the initial density of the pack before it isrolled and the final density of the pack after the rolling operation iscomplete, particularly with regard to the pack width. The foregoingrelationship is referred to herein as the solidity factor, and isexpressed as a percentage ratio of the actual density of materialswithin cavity 42 to the theoretically ideal density of such materials ifcavity 42 were completely filled with workpiece and filler materialshaving the same proportion of relative mass as the materials actuallyused. v

In determining the solidity factor of a pack before rolling the same, itisnecessary for the height of side and end pieces 34-40 comprising theyoke to be substantially uniform. Similarly, the inner surfacesv 35 and37 of side pieces 34 and 36, respectively, of the yoke should besubstantially uniformly planarwhereby the width of cavity 42 will besubstantially constant throughout the total length of the yoke. Bottomenvelope portion 32 may advantageously be joined to the yoke in therelationship shown by FIG. 2 by appropriate means such as welding assuggested by fillet 24 before the contents of the yoke are placedtherewithin.

With the yoke thus formed, workpiece components l0, l2 and 14 arearranged within this yoke together with mandrels 18, the stated elementsbeing sized and arranged to substantially fill the entire enclosedvolume defined by the yoke. During the foregoing step, as each member14, 18, 20 and 22 is placed within the yoke, their volumetricdisplacement must be determined. The length of all the foregoing itemscan quite easily be precisely maintained constant and identical by asimple trimming operation, whereby end members 38 and 40 are snuglycontacted by the mentioned items with substantially uniform force.Similarly, face sheets 10 and 12 are accurately formable by precisiontrimming of the same as necessary to fit closely within yoke members 34,36, 38 and 40 as suggested by FIG. 21. However, the widths of items 14,18, 20 and 22 are not usually maintainable with precise uniformity inevery case due to manufacturing tolerances, creep characteristics, andother causes. For example, when mandrels 18 are formed by cold drawing,as is often the most convenient and economical method of fabricatingsuch tooling items, the widths thereof normally vary between individualmandrels and may even be non-uniform between the opposite ends of eachmandrel. Due to the considerations described above, accuratedetermination of-the volumetric displacement of each separate item l4,18, 20 and 22 'cannot be economically achieved on a mass productionscale by a multitude of individual dimensional measurements at aplurality of locations along the total length of every such item,especially where the workpiece typically comprises from 150 to 300 ribs14 and a like number of mandrels l8 having a length as much as 7 to 10feet to form a single panel.

Accordingly, it is an important feature of the inventive concept in thiscase that the volumetric determination required to solve for thesolidity factor as defined hereinabove for a penal illustrativelycomprising 6AL4V titanium workpiece ribs 14 and steel mandrels 18, 20and 22, is achieved by the following mathematical relationships andequivalents thereof:

Solidity factor (X, X,/X,,) where:

X, is the mean theoretical width of all titanium ribs 14 in cavity 42;X, is the mean theoretical width of all steel mandrels 18, 20

and 22 in cavity 42; and X is the actual measured width of yoke cavity42 between inner surfaces 35 and 37 of side members 34 and 36,

respectively, as seen from FIG. 2, for example. In the foregoingcontext, the mean theoretical width X, of all ribs 14 is computed asfollows:

l r i D) where: I W, the total actual weight of all ribs 14 in pounds;L, the actual length of all ribs 14 in inches (constant);

T, the thickness of the workpiece panel before rolling I the height ofcavity 42, and v D, =the density of the material in ribs 14.

Similarly, the mean theoretical width X, of all steel mandrels 18, 20and 22 is computed as follows:

W.= the total actual weight of all mandrels in pounds;

L, the actual length of all mandrels in inches (constant);

T.=T (see above), and

D,=the density of the material in mandrels 18, 20, 22.

The following example is intended to illustrate use of the foregoingrelationships in fabricating a workpiece panel having a pre-rolling sizeof 48 inches wide by 36 inches long involving assumed values as statedbelow:

Measured height of cavity 42 l .006 inches.

Measured width of cavity 42 X, 48.25 inches.

Total number of mandrels 18 142 plus 2 half-size mandrels 20, 22 143.

Cross-sectional area of mandrels 18 0.236 sq. in.

Total cross-sectional area of mandrels l 8, 20, 22 143 x 0.236=33.8 sq.in. I

Total number of ribs 14 of 6AL4V titanium 143.

' Cross-sectional area of ribs 14 0.0157 sq. in.

Total cross-sectional area of ribs 14 143 X 0.015 7 2.25 sq. in.

Length of mandrels and ribs 36 inches.

2 titanium face sheets 10 and 12, each 48 inches wide and 0.125 inchesthick, total cross-sectional area 48 x -0.l25 2=l2sq.in.

The significance of a close tolerance control such as $0.002 inches isapparent in the area calculation considering the large number of detailparts in the assumed case.

Total actual weight of items l0, l2 and 14 82.1 pounds.

Total actual weight of items 18, 20 and 22 345 pounds.

Since the solidity factor thus computed is less than 99.5 percent, theaddition of material to cavity 42 in the assumed'case is required toachieve the preferred value. It is' a significant feature of the conceptdisclosed herein that the solidity factor comprising the percentagecomputed illustratively hereinabove must be preferably at least 0.995before the pack is closed and the rolling operation begun. If thesolidity factor as defined herein is less than the foregoing value,correction of the condition is necessary, and is most convenientlyaccomplished by the addition of material in the from of one or moreshims along the sides of the yoke cavity 42' as indicated by shim 44 inFIG. 2, for example. The length and thickness of shim 44 in the assumedillustrative case is determined'from the calculations shown above. Thus,since the total volume of cavity 42 is known, shim size is determined bycomputing shim dimensions capable of resulting in the stated value whenadded to the computed volume of workpiece and mandrel materials. Asapplied to the case illustratively assumed above,

since the computed solidity factor of 98.9 percent is less than thepreferred value of 99.5 percent, addition of a shim as required toachieve the-latter value will require a thin steel strip of 36 inchlength, 1.006, inches height, and a width equivalent to, the differencebetween 48.25 inches and 47.75 inches, or 0.50 inch, positioned withincavity 42 as shown by shim 44 in the drawing. After preparation of thepack, and closure of the envelope as by welds 24, evacuation ofatmosphere therefrom may be accomplished by use of a vacuum connec-'tion 62 and the pack is subjected to heating and rolling.

The rolling operation on the assembly shown by FIG. 2, for example,during which the workpiece thickness may be reduced as much as 60percent or more, produces extreme roller compression forces and isaccompanied by elevated heating necessary to produce solid statedifi'usion bonding between ribs 14 and face sheets and 12. Theseoperating conditions simultaneously produce incomplete but nonethelessrather tenacious bonds between the envelope elements 34, 36, 38, 40, 30and 32 as well as mandrels 18, 20, 22 and the contacting surfaces ofelements 10, 12 and 14 adjoining each of the respective envelope andfiller elements. Due to the extremely high order of correspondencebetween actual density to theoretically achievable density of thematerials in cavity 42 of the envelope before rolling, greateruniformity and intensity of surface contact between the envelope andfiller materials and workpiece surfaces results from the methoddescribed above than that associated with the generally haphazardmethods of pack preparation known to the prior art. Accordingly, removalof workpieces from the envelope and of such mandrels from roll diffusionbonded articles in a pack which was prepared using the solidity factormethod described above is a particularly sever problem, withcommensurately high risk of damage to such articles resulting fromapplication of force to remove such items. The leaching of mandrels fromthe finished article by immersion of a pack in a leaching solution afterfabrication of the article is otherwise complete, in the manner familiarto the prior art, is distinctly unsuited to the instant case from aneconomic standpoint due to inability of the liquid bath to penetratebetween the tightly adhering mandrel and workpiece surfaces.

Accordingly, it is an important feature of the inventive concept in thiscase that the magnetic properties of the materials in the pack are usedin the'method for removing mandrels 18, and 22 therefrom. Thus, mildsteel or the like as suggested for filler material is stronglyresponsive to magnetic force, whereas titanium and some of its alloysare magnetically inert in the sense that they are neither attracted norrepelled by magnetic force. As a result of the foregoing relativeproperties between the workpiece and filler materials illustrativelysuggested for practicing the method disclosed herein, removal ofmandrels is accomplished by subjecting the workpiece and fillermaterials contained therein to magnetic force having sufficient fieldstrength to distort the mandrels. While various methods and means may beused to accomplish the stated objective, use of a portable magnetichammer of the general type shown in U. S. Pat. No. 2,976,907 issued Mar.28, 1961, or variations thereof, has been found very effective.

Removal of the mandrels may appropriately begin by first opening thepack by removing upper and lower envelope portions 30, 32, side portions34, 36 and end portions 38, 40 of the envelope yoke. In connection withthe envelope removal step, it is a significant feature of the inventiveconcept in this case that means are provided in the pack prior to therolling operation for facilitating removal of the envelope components.Thus, a plurality of relatively thin foil strips such as strips 46, 48,50 and 52 of titanium are preplaced in the pack during initialpreparation thereof as suggested in FIGS. 1, 2 and 3 so as to overlapthe planes of contact between face sheets 10 and 12 with the innervertical surfaces of yoke members 34, 36, 38 and 40. The foregoingrelationship is best seen, for example, from FIGS. 2 and 3 showing foilstrips 52, 53 preplaced between yoke member 34 and covers 30 and 32 ofthe envelope assembly, respectively. The overlapping relationship thusshown for foil strips 52 and 53 is identical for all the remaining foilstrips with respect to the other envelope components.

The particular advantage in use of titanium foil strips such as 46, 48,50 and 52, for example, is best appreciated by consideration of theresults of the roll diffusion bonding operation without such foilstrips. Thus, referring to FIG. 2, it may be seen that yoke members 34and 36 are permanently joined by a solid state bond to upper and lowerenvelope covers 30 and 32, whereby removal of the workpiece from theenvelope after such rolling operation involves the use of a flamecutting torch or the like to separate the envelope components thussurrounding the workpiece. The cutting line for separating elements 30and32 from yoke element 34, for example, would necessarily have tocoincide substantially with the vertical plane defined by surface 35 inFIG. 2, with the result that molten material from the stated envelopecomponents mingles with molten material from the workpiece elements andfuses them together so that mechanical cutting or grinding after theflame cutting operation is necessary to separate the mentioned parts.This naturally reduces the size of the workpiece and prolongs thefabrication period involved in completing workpiece panels, increasingthe final cost thereof.

The foregoing difficulties are avoided by preplacement of titanium foilstrips such as 52 and 53 shown in FIGS. 2 and 3, for example, it beingunderstood that titanium bonds incompletely with steel and that therolling process discussed herein will provide weaker bonds of titaniumto steel than those produced between steel to steel or titanium totitanium surfaces. Due to the foregoing phenomena, the foil functionsessentially as a stop-off medium. Accordingly, after the rollingoperation is complete, flame cutting may occur along a line generallydefined by line 58 in FIG. 3, for example, which passes through bothfoil strips 52 and 53. Similarly, flame cutting may occur through theremaining yoke members 36, 38, and in the same manner as member 34 shownin FIG. 3. Thereafter, a single hammer blow on a chisel or the likeinserted at the location of each of the foil strips 52 and 53, forexample, is often sufficient to cause cover members 30 and 32 to pop offthe pack assembly in the manner of a spring due to the residual stressestherein remaining after the rolling operation.

Referring to the removal of mandrels from the workpiece, it may be seenfrom the illustrative embodiment suggested by FIG. 5 that all of theenvelope portions have been removed,

. after which magnetic hammer 54 is moved progressively across onesurface of the workpiece in the relationship shown, and is repeatedlyenergized to expose all of the stated surface and underlying componentsto the concentrated local magnetic field produced by coil 56 of hammer54. If necessary or desirable following the use of hammer 54 in theforegoing manner, the workpiece assembly shown in FIG. 5 may be invertedand the magnetic hammering process repeated, whereby the same parts areprogressively subjected to magnetic force from the opposite direction.It will be understood that, in the case of small workpieces orrelatively huge coils, all the required magnetic force may be appliedsimultaneously to all portions of the workpiece assembly rather thanprogressively in the manner described for larger workpiece assembliesand shown in FIG. 5. Moreover, the orientation of flux lines in thefield generated by coil 56 will depend upon the size, shape and massdistribution of the filler material which is sought to be removed, andpreferably should be arranged to produce the maximum possible torque orother distorting force in the mandrels. Thus, flux lines passing throughtitanium face sheet 10 or 12 shown in FIG. 4 by magnetic hammer 54 willresult in slight but definite and critically important distortion andcon sequent relative movement between mandrels 18 and titanium workpieceelements 10, 12 and 14. Such distortion is normally very localized andtemporary in nature, since the resilient properties of mandrels 18, forexample, will restore the mandrels to their undisturbed state afterremoval of magnetic force from coil 56. However, the stated distortionand consequent slight relative movement between mandrels 18 and theworkpiece has been found sufficient to reduce somewhat the holding orretaining force between the workpiece and the mandrels, as a result ofwhich the mandrels are more readily removable from the workpiece panel,and/or more amenable to the pressurizing process step suggestedillustratively by FIG. 6.

Referring to FIG. 6, it may be seen that a leaching step may beaccomplished by immersion of the workpiece shown in FIG. 5 in acontainer of solution adapted to dissolve filler material of mandrelsl8, and that the mandrel ends are exposed to such solution forsufficient time to remove a small portion thereof. Leaching may beaccomplished as known to the prior art and taught by U. S. Pat. No.3,044,160 issued July 17, 1962. The foregoing step results in a cavityat each of the mandrel ends such as cavity 59. Thereafter, the workpiecepanel may be subjected to one or more applications of fluid pressure bysuitable means which may take the form of a manifold as shown bymanifold 70 adapted to cover the end of the workpiece in sealing contacttherewith. End closure member 72 having a suitable pressure line 74connected therewith may be joined to manifold 70 in sealing relationshipor may be integrally formed thereon, together with another end closuremember oppositely corresponding to item 72 for sealing the opposite endof manifold 70 (not shown). Manifold 70 is adapted to cover cavities 59and to be sealed to the confronting end of the workpiece panel bysuitable means such as pressure tape or the like. Similarly, anothermanifold oppositely corresponding to manifold 70 is provided on theopposite end of the workpiece panel and sealed thereto. Thereafter,fluid pressure through line 74 is applied to fill cavities 59 and totraverse the length of the workpiece panel, particularly betweenmandrels 18 and face sheets 10 and 12 thereof, producing gaps 60 shownin FIG. 4, thereby further breaking any remaining bonded areas betweenthe mandrel and workpiece surfaces. The pressure applied through line 74may be varied and/or cycled repeatedly if useful or necessary in anyparticular case to achieve the maximum possible loosening effect on themandrels. Moreover, it will be understood that the pressurizing stepthus suggested by FIG. 6 and discussed above may be separately used withor without the magnetic hammering step described in connection with FIG.5.

In addition, if the leaching process mentioned hereinabove and known tothe prior art is considered necessary or desirable due to high residualstresses in the workpiece panel which continue to grip the mandrels 18in the event that the methods suggested in FIGS. and 6 do notsufficiently loosen the mandrels for removal thereof, the magnetichammering and pressurizing processes thus described have been found toresult in improved capability of leaching fluid to traverse throughoutthe length of the workpiece panel. As a result of either or both themagnetic hammering and pressurizing processes, minute gaps between themandrel and workpiece surfaces generally corresponding to gap 60 arefound to result, although it will be understood that gap 60 in FIG. 4 isexaggerated for the sake of clarity. Depending upon particular workpieceand mandrel sizes and materials, gap 60 may not be continuous or uniformin location throughout the workpiece, but in any case will result in asignificant increase in the ability of the leaching solution topenetrate within the workpiece and to contact a greater total portion ofthe mandrel surface whereby the removal process by leaching is greatlyaccelerated. In this connection, it has been found that immersionperiods required to leach mandrels 18 from a panel of the type shown inthe drawings of this case can be reduced as much as 50 percent or more,resulting in a vastly more economical process for mass production use inthe fabrication of such panels.

In further connection with the improved structural continuity andintegrity resulting from the inventive concept disclosed herein, it is afurther significant feature of this concept that fillets are achieved atall the joints between ribs 14 and face sheets 10 and 12 due to theprovision of excess metal at the ends of ribs 14 and further due to theshaping of mandrels 18, and 22 to provide gaps for accommodating suchexcess metal. Thus, referring to FIG. 3, it may be seen that end portion66 of each rib 14 is slightly longer than necessary to extend betweenface sheets 10 and 12, whereby upper face sheet 10in FIGS. 2 and 3 isshown to be slightly deformed upwardly. Moreover, surface 64 on mandrel18 may be seen to lie in a slightly lower horizontal plane than thatdefined by the top surfaces of the mandrels l8 and 20 on either sidethereof as seen particularly in FIG. 3. The force of compression duringthe rolling operation thus deforms the end portions 66 of ribs 14 andcauses the same to flow into the linear gaps or spaces adjacent to andcoextensive with surface 64 of mandrel l8 and also the gaps 68 resultingfrom the rounded corners of the mandrels as illustratively shown in FIG.3. The avoidance of square edges and sharply defined internal comers inall joints of the workpiece by use of fillets as produced by thementioned mandrel rounded edges, resulting gaps 68, and excess ribmaterials 66, results in improved strength and structural integrity byavoiding the localized stress concentrations which would tend to producecracks or other incipient failures at such comers and edges in afinished workpiece panel.

It will further be understood by those skilled in the art that thesolidity factor method of pack preparation disclosed above is applicableto roll diffusion bonding of workpieces other than the sandwich typepanels illustratively shown in the drawings and discussed above toexplain the novel concept. Thus, the fabrication of reinforced sheetshaving upstanding ribs or T" cross-sectional braces secured to suchsheets without a permanent covering plate can advantageously be achievedby roll diffusion bonding in packs prepared with initial densitiesdetermined according to the teachings set forth herein, either with orwithout the subsequent use of the magnetic hammering and pressurizingmethods discussed above.

While the particular details set forth above and in the drawings arefully capable of attaining the objects and providing the advantageshereinstated, the structure and method thus disclosed are merelyillustrative and could be varied or modified to produce the same resultswithout departing from the scope of the inventive concept as defined inthe appended claims.-

We claim:

1. In a method of forming an article from a plurality of nonmagneticallyresponsive workpiece components and a plurality of magneticallyresponsive filler elements substantially enclosed within said componentsand initially assembled in mutual contacting relationship, the steps of:

subjecting said components and said filler elements to sufficient heatand pressure to bond said components permanently together, and

loosening said filler elements to facilitate their separation from saidworkpiece components by subjecting said assembly to magnetic force in anamount sufficient to distort said filler elements.

2. The method set forth in claim 1 above, wherein:

said workpiece components comprise titanium, and

said filler elements comprise steel.

3. The method set forth in claim 1 above, wherein:

said magnetic force is progressively applied .by a portable magnetichammer moved over an external surface of said assembled components andfiller elements and in close proximity thereto whereby lines of magneticflux pass entirely through said article.

4. The methodset forth in claim 1 above, further including:

the step of applying fluid pressure to said article at an externallocation thereon communicating with said filler elements whereby saidfluid pressure progressively penetrates between confronting surfaces ofsaid filler elements and said workpiece components.

2. The method set forth in claim 1 above, wherein: said workpiececomponents comprise titanium, and said filler elements comprise steel.3. The method set forth in claim 1 above, wherein: said magnetic forceis progressively applied by a portable magnetic hammer moved over anexternal surface of said assembled components and filler elements and inclose proximity thereto whereby lines of magnetic flux pass entirelythrough said article.
 4. The method set forth in claim 1 above, furtherincluding: the step of applying fluid pressure to said article at anexternal location thereon communicating with said filler elementswhereby said fluid pressure progressively penetrates between confrontingsurfaces of said filler elements and said workpiece components.